Field of Application
The present invention relates in general to a direct conversion radio receiver for a FSK (frequency shift keyed) RF (radio frequency) signal, and in particular to a direct conversion radio receiver having an improved circuit for executing 90.degree. phase shifting between an I (in-phase) signal and a Q (phase quadrature) signal generated within such a radio receiver, to enable a demodulated digital signal to be subsequently derived.
In recent years, direct conversion radio receivers for FSK signal reception have been proposed, which have the basic advantage over conventional frequency conversion (i.e. superheterodyne) receivers of being highly applicable to implementation, substantially entirely, as a single integrated circuit. In particular, direct conversion FSK radio receivers have been proposed in which it is only necessary to utilize a single local oscillator circuit, producing an output signal that is held close to the carrier frequency of the FSK signal. Such radio receivers are described in U.S. Pat. No. 4,193,034, and also in U.S. Pat. No. 4,462,107. The basic principles of such a direct conversion FSK radio receiver will be described referring to the latter U.S. patent. The first embodiment of that patent is illustrated in FIG. 1. Here, the input FSK RF signal consists of a carrier of frequency f.sub.c which is modulated by being successively shifted upward or downward in frequency by a fixed amount (that amount being designated as in the following) in accordance with an original digital signal, e.g. in accordance with whether the FSK signal represents a 0 or 1 bit. That RF signal, which can be expressed as f.sub.c + is applied directly to one input of a first mixer circuit 21, and through a 90.degree. phase shift circuit 23 (i.e. which produces a phase shift of 90.degree. at frequencies in the f.sub.c + range) to one input of a second mixer circuit 22. A local oscillator signal generating circuit 24 produces a local oscillator signal at a frequency that is very close to the carrier frequency f.sub.c , which is applied to the respective other inputs of the first and second mixer circuits 21, 22 (although, as described, it is equally possible to apply the 90.degree. phase shift to the local oscillator signal that is supplied to one of the mixers, rather than to the input RF signal). The respective output signals from the mixer circuits 21, 22 are passed through respective low pass filters 25 and 26. The output signals thereby obtained from the low pass filters 25 and 26 each have a frequency that is the difference between the frequencies of the local oscillator signal and the input RF signal, i.e. if the local oscillator frequency were exactly equal to the carrier frequency f.sub.c, each of these output signals would have a frequency that is equal to the frequency shift amount .delta.. The output signal from the low pass filter 26 is passed through a circuit which produces a phase shift of 90.degree. at the frequency of that output signal, i.e. in the baseband frequency range, then is converted to a digital signal by an amplitude limiting amplifier circuit 29. The output signal from the low pass filter 25 is similarly converted to a digital signal by an amplitude limiting amplifier circuit 28. The output digital signals from the amplitude limiting amplifier circuits 28, 29 are then processed in a digital logic circuit 30, to obtain a demodulated digital output signal.
The operation of that circuit is as follows. Due to the 90.degree. phase shift applied to the input of the mixer circuit 22, the output signals from the low pass filters 25, 26 will be in phase quadrature. However each time the direction of frequency shift of the input RF signal changes, i.e. from +.delta. to -.delta. or vice-versa, the phase relationship between these output signals from the low pass filters 25, 26 will change by 180.degree., i.e. the output signal from the low pass filter 27, generally referred to as the phase quadrature or Q signal, will shift in phase by 180.degree. with respect to to the output signal from the low pass filter 25 (generally referred to as the in-phase or I signal). Thus, by phase shifting one of the I and Q signals by a fixed amount of 90.degree., these signals will be either mutually in-phase or 180.degree. out of phase, in accordance with the direction of frequency shift of the input RF signal. Hence, a demodulated digital signal can be derived by digital processing of the I and Q signals. In the simplest case, the digital logic circuit could consist of a single exclusive-OR gate circuit.
If the frequency of the local oscillator signal is precisely identical to the carrier frequency f.sub.c, then such a circuit would be satisfactory. However in a practical manufacturing situation, it is difficult to achieve a sufficiently high degree of accuracy for the local oscillator signal. If the local oscillator signal is higher than the carrier frequency of the FSK RF signal, then the I and Q signals from the low pass filters 25, 26 will each be increased in frequency beyond the theoretical value, while if the local oscillator signal becomes lower in frequency than the carrier frequency, the I and Q signals will be correspondingly lowered in frequency. Thus, it is necessary that the phase shifter 27 be of broad-band type, in order to cope with such frequency errors. Since each of the I and Q signals is a low frequency signal in the baseband frequency region, there will be difficulty in implementing a suitable wide-band 90.degree. phase shifting circuit as the circuit 27, if a conventional analog type of phase shifting circuit is used, particularly when that circuit must be formed within an integrated circuit.
In the following, derivation of the aforementioned I and Q signals from an FSK RF signal will be referred to as quadrature demodulation.
A second problem which arises with a receiver circuit of the form shown in FIG. 1 also results from frequency error of the local oscillator signal. Since the carrier signal is of very high frequency by comparison with the baseband frequency region, even a small amount of error between the local oscillator signal frequency and the carrier frequency f.sub.c will result in a significant change in the frequency of each of the I and Q signals obtained from the low pass filters 25, 26. In particular, if the local oscillator frequency is slightly lower than the carrier frequency, then the frequency of the I and Q signals will be lowered. Each time a logic level transition of the original digital signal (i.e. the signal that is to be demodulated) occurs, there will be a certain amount of delay before the mutual phase relationship between the I and Q signals becomes shifted by 180.degree. as a result of the corresponding frequency shift of the RF carrier. As the frequency of the I and Q signals is lowered, the amount of that delay increases. Hence, for a specific value of data rate of that modulating digital signal (i.e. a specific value of bit period of that signal), there is a lower limit to the frequency of the I and Q signals beyond which processing these signals to obtain the final demodulated digital signal becomes impossible. In practice, the demodulation error rate of such a circuit rapidly increases as the frequency of the I and Q signals approaches that lower limit. Alternatively stated, the maximum anticipated amount of frequency error of the local oscillator signal sets a limit to the maximum data rate of FSK modulation that can be utilized, with the circuit of FIG. 5.
To overcome that second problem, the assignee of that prior art patent has proposed second and third embodiments, in which phase shifting by values other than 90.degree. is executed, in addition to phase shifting by 90.degree., with the various signals thus obtained being processed in combination to obtain the demodulated digital signal, to enable an increase in the maximum data rate (or alternatively, maximum permissible local oscillator frequency error) to be achieved. However such a method does not overcome the first-mentioned problem of the difficulty of implementing wide-band phase shifting circuits operating in the baseband frequency range, and in particular the difficulty of implementing such phase shifting circuits within an integrated circuit.